Washington -LRB- CNN -RRB- -- Pilot Peter Hertzak , a cosmetic surgeon , took off on a solo flight from Slidell Municipal Airport in Slidell , Louisiana , last Friday in his twin-engine Cessna 421C en route to Sarasota , Florida . Some three hours later , his plane descended in a fatal death spiral into the Gulf of Mexico . What happened in between still has federal investigators looking for clues .

The National Transportation Safety Board released a preliminary report on Friday that provided little in the way of explanation as the investigation continues .

Hertzak filed instrument flight rules and departed around 6:45 a.m. local time , the NTSB said .

Investigators say the pilot contacted the Jacksonville Air Traffic Control Center at approximately 7:15 a.m. to report on his flight altitude .

About 20 minutes later , the airplane began to `` deviate from the air traffic control assigned altitude and route . ''

The NTSB said the air traffic control center tried unsuccessfully to contact Hertzak .

The North American Aerospace Defense Command launched military fighter aircraft to intercept the plane . According to the NTSB , the military pilots reported the airplane was `` circling in a left hand turn at a high altitude , low airspeed and the windows were partially frosted over . ''

They reported Hertzak was `` slumped over in the cockpit and not moving . '' The NORAD planes fired flares , but the pilot remained unresponsive .

The Cessna circled for about three hours at about 28,000 feet before it made what appeared to be a soft landing . A Coast Guard spokeswoman said the plane was afloat with its tail sticking out of the water before it soon sank , about 190 miles southeast of Mobile , Alabama .

Three days prior to the incident , Hertzak had been placed on probation and permanently banned from performing plastic surgery , according to Rita Arceneaux , spokeswoman for the Louisiana Board of Medical Examiners .

When asked if the NTSB had any reason to believe the incident could have been a suicide , NTSB spokesman Eric Alleyne told CNN that was `` beyond the scope of my investigation . ''

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Safety board report offers no explanation of what caused pilot Peter Hertzak 's fatal crash

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During Louisiana to Florida flight , his plane went off course , controllers could n't contact him

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Military pilots sent to investigate reported Hertzak was slumped over as plane circled

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The Cessna spiraled for three hours before an apparent soft landing in Gulf of Mexico